Large capital investments are typically required to produce any oil reservoir, and much of that investment is in the construction of deep wells which are located in the very part of the reservoir that is of greatest interest to characterize and monitor, i.e. where the oil is. One of the primary goals, therefore is to improve recovery efficiency for existing resources because the cost of developing new fields is increasingly expensive. This is accomplished by deriving useful information about field production.
In the prior art, seismic tomography, which performed from the surface only, or conventional borehole geophysics has been used. However, moving sondes in boreholes for logging or crosshole tomography, or moving sources and receivers on the surface for reflection seismology, are time consuming and expensive operations. For example, the cost of a 3D seismic survey can reach $1 million or more. Conventional borehole geophysics is less expensive but has an upfront cost and a downtime cost. Additionally, conventional borehole techniques tend to have a narrow filed of view. For example, borehole logging is focused on a narrow strip around the well bore. Similarly, seismic crosshole tomography is insensitive to all but a narrow region directly between the well bores. Alternatively, prior art practices have utilized sensors which were placed inside the casings, which prevented operation of oil recovery operation during that monitoring/sensing period. In any of these monitoring methods, the time interval between surveys is generally limited to the survey costs and the reluctance to remove wells from production due to downtime costs.
Because sensors placed at these locations are thereby nearest to the volume of interest and most sensitive to the reservoir and the processes resulting in oil production, there is a need for placing sensors deep in oil reservoirs, and a need to monitor critical parameters, e.g. geophysical data, in an oil reservoir to provide knowledge of the reservoir and related processes such as primary and secondary recovery, but in a manner which does not affect production operations. Therefore there is a need for a monitoring tool capable of providing low-cost, long-term, near-continuous imaging, while having minimum impact on production operations, and not limited by mobilization costs, survey costs, downtime costs, or demobilization costs.